Fix restoring stderr correctly

When running `ocaml-mdx` it would output warnings to the "corrected"
files when running with `--output -`. Since warnings are emitted on
stderr this should not be happening, which means that something was
messing with stderr. It turns out yes, there is code which modifies
stderr but also takes care of restoring it. Unfortunately a typo caused
the backup to be stderr instead so restoring would redirect stderr to
stdout.

Author: Leonidas-from-XIV

lib/top/mdx_top.ml

@@ -22,7 +22,7 @@ type directive = Directory of string | Load of string
 
 let redirect ~f =
   let stdout_backup = Unix.dup Unix.stdout in
-  let stderr_backup = Unix.dup Unix.stdout in
+  let stderr_backup = Unix.dup Unix.stderr in
   let filename = Filename.temp_file "ocaml-mdx" "stdout" in
   let fd_out =
     Unix.openfile filename Unix.[ O_WRONLY; O_CREAT; O_TRUNC ] 0o600